In the production of surface coverings, particularly decorative surface coverings and more particularly floor coverings, multiple coating layers are desirable to add different features, such as design and/or color elements and durability (wear) to the product. A decorative surface covering typically includes one or more special coatings, each of which is designed to provide a desired mechanical strength and/or decorative effect. These coatings are typically applied one at a time, with each coating being separately applied and cured before adding an adjacent coating layer. Such a procedure has been deemed necessary to maintain a distinct relationship between the separate layers and to prevent mixing of the coatings or contamination of one coating by another at the interface of the layers. These layers are typically applied by reverse roll coaters, forward roll coaters, blade over roll coaters, air knife coaters and other application methods known in the art. Certain coating layers, such as those containing abrasive materials, may need to be applied by a different method than other coating layers, requiring different coating application equipment in the production line. The multiplicity of coating applicators and curing stations is costly, requiring much capital, building space, and time to produce a single decorative surface covering product.
Where surface coverings or surface covering components are manufactured by individually laying down multiple layers on a substrate, each layer is laid down separately by passage through a roll coater, then fused, gelled or cured by processing over massive heated drums and/or in long forced air ovens. To lay down several layers, multiple passes of the goods through several production lines is required. Thus, the process is costly due to the high capital cost of roll coaters and factory space needed to house them. Further, scrap loss is incurred due to multiple passes through the rollers.
Roll coating also limits the types of coatings that may be applied to a support because roll coaters are inherently sensitive to the rheology of the applied liquid layers. High viscosity materials often cannot be used. This makes it difficult to use high performance wear-layer coatings that include high molecular weight, high viscosity materials. Additionally, add-on equipment to monitor and control the roll coaters is required to control the application thickness of each layer.
Slot die coating is a continuous coating technique which delivers quantitatively precise amounts of a material, typically of low solids and viscosity characteristics, to an applicator which deposits quantitatively precise amounts of the material on a traveling web or other substrate through an opening or slot in the applicator through which the fluid material exits. Typically, slot die coating is limited to smooth, nonporous surfaces such as photographic films, papers and circuit boards, and coatings including non-interactive chemistries. The use of slot die coating in photographic films allows coating layers of low viscosity films in thicknesses of one micron or less to be applied to a substrate. See, for example, U.S. Pat. Nos. 2,761,417 and 5,143,758. Typically a slide or cascade slot die coater is used to apply a plurality of wet on wet layers for photographic films. Slot die coaters have also been used for the manufacture of high performance composite membranes, which have a total thickness of less than 1 micron, and include multiple layers applied sequentially to a substrate. See, for example, U.S. Pat. No. 6,132,804. Further, slot die coating has been used to apply adhesive coatings to a substrate, as illustrated, for example, in U.S. Pat. Nos. 5,728,430, 5,871,585 and 5,962,075. Typically, the coating layers are applied at the tangent of the substrate roll and the slot die opening. Slot die coating has not been used for the application of viscous and/or high molecular weight materials, such as plastisols.
It would be advantageous to provide a method for reducing the capital, building space and time required to produce a decorative surface covering product. It would further be advantageous to provide a method for applying high solids and high viscosity coating layers, each with its own specific thickness, which does not require additional control measures. The present invention provides such methods.